In the thermal cracking of distillation residues or heavy gas oil fractions obtained in the processing of crude oil, petroleum coke is formed which is separated in coking chambers and cut out by means of pressure water jets.
The crude coke, which is also called green coke, still contains 6-15% of heavy, asphalt-type hydrocarbons and 12-20% water depending on the duration of storage.
The coke is primarily used for the production of electrodes. However, this production requires the removal of the water and the hydrocarbons from the green coke, for which purpose the green coke is heated to temperatures within the range of 1200.degree.-1400.degree. C., which reduces the specific electric resistance of the petroleum coke from 3.7.times.10.sup.6 .OMEGA./cm to 0.014 to 0.016 .OMEGA./cm. The coke is converted by an isolator into an electric conductor.
According to the present state of the art, the calcining is carried out in rotating hearth-type furnaces or in cylindrical rotary kilns, but discontinuously operated pit furnaces are also in use. However, the number of cylindrical rotary kilns substantially exceeds the number of rotary-table installations. For production capacities in excess of 500 tons/day, cylindrical rotary kilns are exclusively used. On the whole, the cylindrical rotary kiln has an advantage over the rotary-table furnace inasmuch as it allows for more processing variations, so that the manifold, continuously growing demands upon the quality of the calcinate may be satisfied.
The following description thus relates to the use of a cylindrical rotary kiln according to the present state of the art.
The wet green coke is placed into the cylindric rotary kiln which is slightly inclined. The charge is slowly conveyed through the furnace by the rotation of the kiln. At the product discharge side, the kiln is heated from the head of a gas or oil burner; the hot gases flowing countercurrent to the coke. The hydrocarbons expelled from the coke are partially burnt in the kiln and as a result supply much of the energy required in the process. The calcinate, under temperatures of from 1150.degree. to 1350.degree. C., drops into a rotary cooling drum where it is quenched with water. The off-gas escaping on the product inlet side of the kiln still contains combustible components (CO, H.sub.2, hydrocarbons) and large quantities of coke dust. The gas has a temperature of from 500.degree. to 800.degree. C. depending on how the kiln is operated and on the water content of the charged material. This gas is called lean gas.
The treatment of said lean gas constitutes an important part of the entire process. In view of the environment protection provisions which are becoming increasingly stricter, a satisfying and economically acceptable solution to this problem will determine whether continuous petroleum coke calcining methods can also be carried out in the future.
Various methods are in use (see AUFBEREITUNGSTECHNIK No. 10/1972--"Die Entwicklung von grossen Drehrohrofen mit Nachbrennern zum Kalzinieren von Koks" (The Development of Large Cylindric Rotary Kilns with After-Burners for the Calcining of Coke)) comprising treating the lean gas by air and fuel admission in combustion chambers connected downstream from the cylindrical rotary kiln, in which chambers the combustible gases and a major portion of the dust are burnt. However, the off-gases even then contain enough remaining dust requiring an intensive separation of this dust by means of filters. This requirement is made difficult by the high temperature of the off-gas in excess of 600.degree. C. Even when using the lean gas in the combustion chamber of a boiler in which the energy of the lean gas is utilized in combination with gas or oil burners for producing steam, the fine de-dusting of the off-gas exiting the combustion chamber at temperatures from 400.degree. to 500.degree. C. remains problematic. The gas volume, due to the temperature, is still very large, and the material of the largevolume filter must meet very high requirements, so that such an installation is expensive both in terms of equipment needed and operation costs. The cooling of the hot off-gases to a temperature permitting purification of the off-gas in textile hose filters is expensive and unprofitable. It is for this reason that installations have also been built in which a portion of the heat of the off-gas is used for drying the wet green coke. This procedure comprises withdrawing 20-40% of the hot boiler off-gases by means of a blower and contacting said gases directly and intensively with the green coke in a rotary drum. The gas leaving the rotary drum in such a direct drying process contains the vapors and has a temperature of 140.degree.-170.degree. C. This measure allows a reduction of the volume of the hot boiler off-gas if both gas streams are de-dusted separately, or the temperature of the total off-gas decreases to 280.degree.-350.degree. C. if the off-gas from the dryer is again admixed with the boiler gas.
The major disadvantage of the direct drying of green coke lies in the fact that the boiler off-gas used for such drying contains large amounts of green coke dust downstream of the dryer. This dust has the dangerous property that it may ignite under certain conditions even at temperatures of less than 100.degree. C. The separation of said dust is highly dangerous because if failures or breakdown occur in the total system, air cannot be prevented from getting into the large-volume off-gas system, and this may cause fires or explosions in the off-gas conduits and the dust filter.
Although the dust problem has remained unsolved in connection with the use of direct green coke dryers, it was found that the use of dried green coke in the calcining kiln offers advantages for the overall controlling of the process in the kiln. The cylindrical rotary kiln may be shorter because if wet green coke is used, about 40% of the length of the kiln is required for the drying.
The temperature distribution in the kiln may be more readily controlled and maintained constant, a factor that has an important bearing on the quality of the calcinate. Furthermore, the use of dried green coke largely prevents the coke from caking upon the lining of the kiln within the inlet zone of the cylindrical rotary kiln. Such caked materials break off from time to time, causing material displacements within the kiln which considerably interfere with the calcining process and result in variations of the quality of the calcinate product.
An additional problem encountered in connection with hitherto employed calcining methods is the cooling of the hot calcinate. The direct cooling of the calcinate in a rotary drum by quenching with water produces substantial amounts of steam (about 10,000 nm.sup.3 per 10 tons of calcinate) containing considerable amounts of dust (about 800-1000 mg/nm.sup.3). The dust separation capabilities of cyclone dust separators as used up until the present time no longer satisfy recent environmental protection regulations. In a wet dust removal step, the reuse of the separated water containing calcinate dust would pose problems. Therefore, plants have been built in which the dust is removed jointly from both the vapors and the boiler off-gases in a central dust separation installation. This measure has the drawback that the vapor quantity varies considerably, which means that it is not possible to avoid pressure fluctuations within the entire flow passage or path of the gas. Pressure variations within the cylindrical rotary kiln, however, constantly cause changes in the temperature distribution within the kiln, and these changes have a negative bearing on the quality of the calcinate. Furthermore, said pressure variations cause vapors to enter into the chute or passageway between the kiln and the cooler, where said vapors react with the red-hot coke to form water gas especially if the product drops from the kiln with temperatures in excess of 1250.degree. C. The subsequent combustion of the water gas severely damages the material by overheating within the product discharge zone of the kiln, the chute or passageway between kiln and cooler and the inlet zone of the cooler.
Furthermore, the water spray-nozzle system in the cooling drum was found to be susceptible to trouble and required considerable maintenance work.
Moreover, in view of the increasing demands relative to the quality of the calcinate, it is disadvantageous that the ash content in the calcinate increase depending on the salt content in the cooling water used. The rate of consumption or burningoff of electrodes manufactured from the calcinate is negatively influenced by the ash content and especially by the calcium content.
It is for these reasons that plants were also built in which the calcinate was cooled indirectly. The material drops into a rotary drum which rotates in an open water bath. The cooling water is maintained at a constant temperature by controlled cooling water admission and withdrawal. This type of indirect cooling has advantages over the direct cooling method; its drawbacks lie in the field of construction materials, corrosion, and a small heat exchange area.